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Because Earth’s magnetic field gradually but continuously changes, the direction of north indicated by a compass also changes. For many reasons, researchers want to know the direction of north at specific times in the past, but finding historic records of compass readings is rare. However, certain paintings can help. For example, these murals in a hall of the Vatican (Bibliotheca Apostolica Vaticana) faithfully recorded the direction of north when they were painted in 1740.

Fig. 32-1 Some of the inks used for tattoos contain magnetic particles.

32-1 What Is Physics?

This chapter reveals some of the breadth of physics because it ranges from the basic science of electric and magnetic fields to the applied science and engineering of magnetic materials. First, we conclude our basic discussion of electric and magnetic fields, finding that most of the physics principles in the last 11 chapters can be summarized in only four equations, known as Maxwell’s equations.

Second, we examine the science and engineering of magnetic materials. The careers of many scientists and engineers are focused ...

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